Digital versatile discs (DVDs) and Blu-ray (registered trademark) discs (BDs) are widely used as information recording media for recording various content such as movies and music.
Copyrights, distribution rights, and the like are held by creators or distributors of many content such as music data, image data, and the like recorded in such information recording media. Therefore, when content is stored in such information recording media and provided to users, it is common to perform usage control such that only users having a legitimate right to use are permitted to use content.
Specifically, for example, control is performed such that content is recorded as encrypted content to be decryptable only by an encryption key provided to users who have performed a legitimate content purchase process. However, although such process is performed, for example, if the user who has obtained the encrypted content performs a process of illegally distributing or releasing the decrypted content or the encryption key, unspecified content is illegally used. Particularly in recent years, there are many cases in which illegal release or delivery of data via a network is performed, and it is a big problem how to prevent such illegality.
As a countermeasure for preventing circulation of illegal content, a configuration capable of identifying a device that has performed a decryption process on the basis of decrypted (plain text) content is known.
This is a configuration of analyzing decrypted content generated by decrypting encrypted content, for example, decrypted image data and identifying a device that has performed a decryption process on the basis of identification data extracted from an image.
A configuration of enabling such source tracking is disclosed in, for example, Patent Document 1 (Japanese Patent Application Laid-Open No. 2006-236121), Patent Document 2 (Japanese Patent Application Laid-Open No. 2007-43336), and the like.
In the configurations disclosed in Patent Documents mentioned above, segments constituting content, for example, images of a scene constituting movie content are set as a plurality of piece of variation data that can be decrypted by different keys. Each reproducing device selects, decrypts, and reproduces one piece of decryptable data from a plurality of pieces of variation data by applying a reproducing device-specific key stored in the reproducing device. The reproducing device in which different keys are stored decrypts and reproduces different variation data for the images of the same scene. As described above, different variation data is selected and reproduced by each reproducing device. In other words, a reproducing process according to a different reproduction path is performed in accordance with the reproducing device.
For example, when copy data of decrypted content is illegally distributed via a network, it is possible to identify a device that has generated the copy data which is illegally distributed, that is, a device that has decrypted the content in predetermined units by analyzing variation data included in the content or a reproduction path.
A range of identifiable devices depends on the number of set reproduction paths. The reproduction path differs in accordance with, for example, a set of encryption keys (decryption keys) (a device key set) stored in each reproducing device. The device key set can be set differently in various units such as device manufacturer units, device type units, individual device units, and the like.
When the device key set is different, it is possible to set different reproduction paths, and in the case of a configuration in which more reproduction paths can be set, it is possible to identify a source of illegal copy content in a narrower range. However, when setting a large number of variation data, the reproducing device needs to select one variation data from a large number of variation data in the content reproducing process.
The reproducing device selects, decrypts, and reproduces decryptable variation data using a key specific to its own device (device key), but when the number of variations increases, if a possibility of decrypting is inspected for all individual pieces of variation data, a processing time increases, and a reproduction delay is likely to occur.